The term "superconductivity" is applied to the phenomenon of immeasurably low electrical resistance exhibited by materials. Until recently superconductivity had been reproducibly demonstrated only at temperatures near absolute zero. As a material capable of exhibiting superconductivity is cooled, a temperature is reached at which resistivity decreases (conductivity increases) markedly as a function of further decrease in temperature. This is referred to as the superconducting transition temperature or, in the context of superconductivity investigations, simply as the critical temperature (T.sub.c). T.sub.c provides a conveniently identified and generally accepted reference point for marking the onset of superconductivity and providing temperature rankings of superconductivity in differing materials. The highest temperature at which superconductivity (i.e., zero resistance) can be measured in a material is referred to as T.sub.o.
It has been recently recognized that certain rare earth alkaline earth copper oxides exhibit superconducting transition temperatures well in excess of the highest previously known metal oxide T.sub.c, a 13.7.degree. K. T.sub.c reported for lithium titanium oxide. Some heavy pnictide mixed alkaline earth copper oxides also exhibit superconducting transition temperatures well in excess of the highest previously accepted reproducible T.sub.c, 23.3.degree. K. for the metal Nb.sub.3 Ge.
Recent discoveries of higher superconducting transition temperatures in rare earth alkaline earth copper oxides are reported in the following publications:
P-1 J. G. Bednorz and K. A. Muller, "Possible High T.sub.c Superconductivity in the Ba-La-Cu-O System", Z. Phys. B.--Condensed Matter, Vol. 64, pp. 189-193 (1986) revealed that polycrystalline compositions of the formula Ba.sub.x La.sub.5-x Cu.sub.5 O.sub.5(3-y), where x=1 and 0.75 and y&gt;0 exhibited superconducting transition temperatures in the 30.degree. K. range.
P-2 C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, and Y. Q. Wang, "Evidence for Superconductivity above 40K in the La-Ba-Cu-O Compound System", Physical Review Letter, Vol. 53, No. 4, pp. 405-407. Jan. 1987, reported increasing T.sub.c to 40.2.degree. K. at a pressure of 13 kbar. At the end of this article it is stated that M. K. Wu increased T.sub.c to 42.degree. K. at ambient pressure by replacing Ba with Sr.
P-3 C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, and Z. J Huang, "Superconductivity at 52.5K in the Lanthanum Barium Copper Oxide System", Science Reports, Vol. 235, pp. 567-569, Jan. 1987, a T.sub.c of 52.5.degree. K. for (La.sub.0.9 Ba.sub.0.1).sub.2 CuO.sub.4-y at high pressures.
P-4 R. J. Cava, R. B. vanDover, B. Batlog, and E. A. Rietman, "Bulk Superconductivity at 36K in La.sub.1.8 Sr.sub.0.2 CuO.sub.4 ", Physical Review Letters, Vol. 58, No. 4, pp. 408-410, Jan. 1987, reported resistivity and magnetic susceptibility measurements in La.sub.2-x Sr.sub.x CuO.sub.4, with a T.sub.c at 36.2.degree. K. when x=0.2.
P-5 J. M. Tarascon, L. H. Greene, W. R. McKinnon, G. W. Hull, and T. H. Geballe, "Superconductivity at 40K in the Oxygen Defect Perovskites La.sub.2-x Sr.sub.x CuO.sub.4-y ", Science Reports, Vol. 235, pp. 1373-1376, Mar. 13, 1987, reported title compounds (0.05&lt;X &lt;1.1) with a maximum T.sub.c of 39.3.degree. K.
P-6 M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, "Superconductivity at 93K in a New Mixed Phase Y-Ba-Cu-O Compound System at Ambient Pressure", Physical Review Letters, Vol 58, No. 9, pp. 908-910, Mar. 2, 1987, reported stable and reproducible superconducting transition temperatures between 80.degree. and 93.degree. K. at ambient pressures for materials generically represented by the formula (L.sub.1-x M.sub.x).sub.a A.sub.b D.sub.y, where L=Y, M=Ba, A=Cu, D=O, x=0.4, a=2, b=1, and y&lt;4.
The experimental details provided in publications P-1 through P-6 indicate that the rare earth alkaline earth copper oxides prepared and investigated were in the form of cylindrical pellets produced by forming a mixed oxide by firing, grinding or otherwise pulverizing the mixed oxide, compressing the particulate mixed oxide formed into cylindrical pellets, and then sintering to produce a polycrystalline pellet. While cylindrical pellets are convenient articles for cooling and applying resistance measuring electrodes, both the pellets and their preparation procedure offer significant disadvantages to producing useful electrically conductive articles, particularly articles which exhibit high conductivity below ambient temperature--e.g., superconducting articles. First, the step of grinding or pulverizing the amorphous oxide on a commercial scale prior to sintering is both time and energy consuming and inherently susceptible to material degradation due to physical stress on the material itself, erosion of grinding machinery metal, and handling. Second, electrically conductive articles rarely take the form of pellets. Electrically conductive articles most commonly take the form of flexible elongated conductive articles--e.g., wires, and articles forming conductive pathways on a substrate, such as insulative and semiconductive substrates--e.g., printed and integrated circuits.
H. Maeda, Y. Tanaka, M. Fukutom, and Y. Asano, "A New High T.sub.c Superconductor Without a Rare Earth Element", Japanese Journal of Applied Physics, Vol. 27, No. 2, pp. L209 and L210, discloses that at least one compound of bismuth, strontium, calcium, copper, and oxygen has been found to be superconducting. More detailed publications in the technical literature on this subject have occurred subsequent to the effective filing date of this patent application. Related Commonly Assigned Co-Pending Applications.
Mir et al U.S. Ser. No. 46,593, filed May 4, 1987, titled CONDUCTIVE ARTICLES AND PROCESSES FOR THEIR PREPARATION, now issued as U.S. Pat. No. 4,880,770, discloses articles containing an electrically conductive rare earth alkaline earth copper oxide layer and processes for their preparation. U.S. Ser. No. 329,049, filed Mar. 27, 1989, is a division of this patent application.
Strom et al U.S. Ser. No. 68,391, filed July 1, 1987, titled CONDUCTIVE THICK FILMS AND PROCESSES FOR FILM PREPARATION, now U.S. Pat. No. 4,908,346 discloses articles containing an electrically conductive rare earth alkaline earth copper oxide layer having a thickness of at least 5 .mu.m and processes for their preparation. U.S. Ser. No. 327,472, filed Mar. 23, 1989, is a division of this patent application.
Agostinelli et al U.S. Ser. No. 85,047, filed Aug. 13, 1987, titled BARRIER LAYER CONTAINING CONDUCTIVE ARTICLES now abandoned discloses articles as described above including a barrier layer interposed between the substrate and conductive layer containing a metal in its elemental form or in the form of an oxide or silicide chosen from the group consisting of magnesium, a group 4 metal, or a platinum group metal. U.S. Ser. No. 330,409 is a continuation of this application and a continuation in part of U.S. Ser. No. 47,593, now abandoned cited above.
Hung et al U.S. Ser. No. 153,699, filed Feb. 8, 1988, titled BARRIER LAYER ARRANGEMENT FOR CONDUCTIVE LAYERS ON SILICON SUBSTRATES now U.S. Pat. No. 4,908,348 discloses articles having a silicon substrate and a conductive layer as described above additionally including a barrier layer triad consisting of silica as a first layer, a group 4 heavy metal oxide as a third layer, and a second layer interposed between the first and third layers of mixed composition. U.S. Ser. No. 326,813, filed Mar. 21, 1989, is a division of this patent application.
Lelental et al, U.S. Ser. No. 208,707, filed June 20, 1988 now abandoned titled IMPROVED HIGH T.sub.c SUPERCONDUCTING Y-Ba-Cu-O THIN FILMS, discloses articles containing an electrically conductive rare earth alkaline earth copper oxide film having a molecular ration Y.sub.1 :Ba.sub.2 :Cu.sub.3 formed over a crystalline Y.sub.2 :Ba.sub.1 :Cu.sub.1 and processes for making such articles.
Agostinelli et al, U.S. Ser. No. 208,706, filed June 20, 1988 titled CONDUCTIVE ARTICLES AND PROCESSES FOR THEIR PREPARATION discloses articles containing conductive rare earth alkaline earth cuprates and heavy pnictide mixed alkaline earth cuprate films having a chemical etch release layer, e.g., CuO interposed between the substrate and the conductive film precursor to improve patterning of the films.
Lelental et al, U.S. Ser. No. 236,420, filed Aug. 25, 1988, titled YTTRIUM RICH CONDUCTIVE ARTICLES AND PROCESSES FOR THEIR PREPARATION, now abandoned discloses articles containing a crystalline conductive film containing a mixture of R.sub.1 A.sub.2 C.sub.3 and R.sub.2 A.sub.1 C.sub.1 phases.
Chatterjee et al, U.S. Ser. No. 290,670, filed Dec. 27, 1988, titled A SUPERCONDUCTIVE CERAMIC OXIDE COMBINATION, discloses passivant polymers for protecting a superconductive ceramic oxide from degradation.
Strom, U.S. Ser. No. 291,921, filed Dec. 29, 1988, titled SUPERCONDUCTING THICK FILMS FOR HYBRID CIRCUITRY APPLICATIONS, discloses a process of forming a thick film crystalline heavy pnictide mixed alkaline earth copper oxide electrical conductor and compositions for their preparation.
Mir et al U.S. Ser. No. 308,297, filed Feb. 9, 1989, titled ELECTRICALLY CONDUCTIVE ARTICLES AND PROCESSES FOR THEIR FABRICATION, discloses a flexible electrically conductive article comprised of an organic film, a conductive crystalline cuprate layer, and a release layer.
Agostinelli et al U.S. Ser. No. 347,600, filed May 4, 1989, titled PROCESSES OF FORMING CONDUCTIVE FILMS AND ARTICLES SO PRODUCED, discloses that the incorporation of silver in concentrations of from 1 to 20 percent, preferably 5 to 10 percent, based on copper in a bismuth mixed alkaline earth copper oxide prior to sintering promotes crystal growth when forming a thin conductive layer on a substrate.
Agostinelli et al U.S. Ser. No. 347,607, filed May 4, 1989, titled PROCESSES OF FORMING CONDUCTIVE FILMS AND ARTICLES SO PRODUCED, discloses that the incorporation of lead oxide in concentrations of from 1 to 50 percent, preferably 5 to 40 percent, based on bismuth and lead in a bismuth mixed alkaline earth copper oxide prior to sintering promotes crystal growth when forming a thin conductive layer on a substrate.
Lelental et al U.S. Ser. No. 347,604, filed May 4, 1989, titled PROCESSES OF FORMING CONDUCTIVE FILMS AND ARTICLES SO PRODUCED, discloses that the proportion of a thin film accounted for by the 37 .ANG. phase can be increased by increasing the proportion of bismuth or bismuth in combination with lead in a bismuth mixed alkaline earth copper oxide to satisfy the metal ratio: EQU M:Cu is&gt;1.00
where M is bismuth and 0 to 50 percent lead, based on lead and bismuth combined.